Catalytic converter with two or more honeycomb bodies in a casing tube and method for its production

ABSTRACT

A catalytic converter includes a casing tube having a given internal diameter. At least two individual honeycomb bodies through which a fluid can flow in a given flow direction, are disposed in the casing tube. The honeycomb bodies are formed of structured metal layers forming flow channels. The layers are joined to the casing tube by joining techniques, such as hard brazing. The honeycomb bodies have a given theoretical strain-free diameter and have an internal layout making the honeycomb bodies elastic and permitting the honeycomb bodies to be inserted into the casing tube with an elastic compression of from 2 to 10% of the given theoretical strain-free diameter and with prestress. A method for producing a catalytic converter includes forming at least two honeycomb bodies with a given theoretical strain-free diameter from structured metal layers defining channels through which a fluid can flow in a given direction. The honeycomb bodies are subsequently successively or simultaneously inserted with prestress from at least one side into a prefabricated casing tube having a given internal diameter. The honeycomb bodies are elastically compressed by from 2 to 10% of the given theoretical strain-free diameter and/or at least partial regions of the casing tube are subsequently plastically compressed by from 2 to 10% of the given internal diameter. The layers are joined to the casing tube.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of International Application SerialNo. PCT/EP93/03057, filed Nov. 2, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalytic converter having two ormore honeycomb bodies disposed in a casing tube, and to a method for itsproduction.

Catalytic converters are used in particular to clean exhaust gases ofinternal combustion engines and are used primarily in motor vehicles. Inorder to achieve a large surface area, such converters generally includeone or more honeycomb bodies that have many channels through which anexhaust gas can flow. Since such honeycomb bodies cannot be producedmonolithically with an arbitrarily great volume, it has long been knownto dispose a plurality of such honeycomb bodies one after the other in acasing tube. An interstice or intermediate space remaining between suchhoneycomb bodies can bring about an additional mixing under somecircumstances and therefore can achieve additional advantages.

Although two or more honeycomb bodies in a single casing tube are shownin a great number of references, such as German Published,Non-Prosecuted Application DE 39 39 490 A1, which is the point ofdeparture for the present invention, nevertheless until now it washardly a trivial problem to economically manufacture such catalyticconverters on a large scale. For various reasons to be discussed indetail below, catalytic converters with two or more disks were thereforefirst assembled (usually welded) at the very end of the manufacturingand coating process, so that the advantages of an integrated productionand only a single part could not be fully exploited.

One reason is that producing a catalytic converter requires relativelymany manipulation steps. It is not only necessary for the honeycombbodies to be inserted into a casing tube, but various steps are alsoneeded for applying brazing material, brazing, and later coating.Typical spirally wound honeycomb bodies, of the kind described in GermanPublished, Non-Prosecuted Application DE 39 39 490 A1, have a tendencywhen axially strained toward mutual displacement of the winding layers,known as telescopes, before they are finally brazed together. Axialforces must therefore be avoided during production. Moreover, brazing inan upright position is not possible, because the axial forces of gravitythat then arise necessarily cause a shift in position. Accuratepositioning of the honeycomb bodies in the casing tube is not easilypossible, either. Honeycomb bodies wound spirally from smooth andcorrugated sheet-metal layers are not very elastic, because the smoothsheet-metal layers extend virtually in a circle, and therefore theelastic forces to be brought to bear by such a honeycomb body areinadequate to absorb the axial forces that occur in high-speedproduction processes.

However, from the prior art, namely Published European Applications No.0 430 945 B1 and No. 0 279 159 B1, other forms of honeycomb bodies arealso known, which have increased elasticity because of their specialconstruction.

Published European Application No. 0 454 712 B1 also discloses honeycombbodies with microstructures that extend transversely to the flow andthat besides influencing the flow also bring about hooking together ofthe foils, so that a kind of form-locking connection between the layersis created that prevents mutual shifting. A form-locking connection isone which connects two elements together due to the shape of theelements themselves, as opposed to a force-locking connection, whichlocks the elements together by force external to the elements.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a catalyticconverter with two or more honeycomb bodies in a casing tube and amethod for its production, which overcome the hereinafore-mentioneddisadvantages of the heretofore-known products and methods of thisgeneral type and which can be produced from the very outset with two ormore honeycomb bodies in one casing tube, even under the conditions ofhigh-speed, large-scale mass production.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a catalytic converter, comprising acasing tube having a given internal diameter; at least two individualhoneycomb bodies through which a fluid can flow in a given flowdirection, the honeycomb bodies being disposed in the casing tube; thehoneycomb bodies being formed of structured metal layers forming flowchannels, the layers being joined to the casing tube by joiningtechniques, such as hard brazing; and the honeycomb bodies having agiven theoretical strain-free diameter and having an internal layoutmaking the honeycomb bodies elastic and permitting the honeycomb bodiesto be inserted into the casing tube with an elastic compression of from2 to 10% of the given theoretical strain-free diameter and withprestress.

The invention assumes that the accelerations which occur in high-speedproduction processes and the later upright brazing of honeycomb bodies,which is desired for various technical and space reasons, are possiblyonly if the honeycomb bodies are entirely fixed in a casting tube byconsiderable elastic forces, and if the individual sheet-metal layerscan no longer shift among one another. In order to bring such highelastic forces to bear, the honeycomb bodies must be compressed by atleast two and up to 10% of their theoretical strain-free diameter. Thetheoretical strain-free diameter is understood to mean the diameter thatthe honeycomb body would assume upon being densely wound or laminated,because of the structures of its metal sheets or layers, if nocompressive forces are exerted upon it. Naturally, a honeycomb body canalready be deformed during production by exerting compressive forces, sothat during the production process it never actually assumes thistheoretical strain-free diameter. It is important, however, that due toits construction the honeycomb body will in fact permit elasticcompressions on this order of magnitude. To that end, either themore-elastic constructions known from the prior art or specialcorrugation forms for the individual layers must be used. Honeycombbodies that include many metal sheets or layers being intertwined withone another, or honeycomb bodies in which sheet-metal layers ofdiffering corrugation but no spiral, smooth sheet-metal layers, areused, are specially suitable for these purposes. The elastic forces thusattained are even sufficient for a residual elasticity to be attained ina high-temperature vacuum brazing process which is high enough to ensurethat the honeycomb bodies can be brazed while standing upright. Althoughsome of the elastic deformation in this brazing process can change intoplastic deformation, nevertheless some residual elasticity alwaysremains, which prevents the honeycomb bodies and layers from shiftingrelative to one another during the vibration-free brazing process. Whileit has already been possible to braze individual honeycomb bodiesdisposed in a casing tube in an upright position, since the honeycombbodies can be supported from beneath, this problem can only be solvedwhen there are two or more honeycomb bodies in a casing tube by usingsufficiently high fixation forces, since not all of the honeycomb bodiescan be supported simultaneously from below.

In accordance with another feature of the invention, the effect broughtabout by the elastic forces is additionally reinforced by certainconstructions or internal form-locking connections. Internalform-locking connections in particular prevent axial shifting ofspirally wound layers, in which such shifting otherwise especiallyoccurs. The use of metal sheets or layers wound in an S or of honeycombbodies with many otherwise intertwined sheets or layers also helps toavoid axial shifting of the layers, since the sheets or layers arerelatively short and rest by their ends on the casing tube, so that theshifting is prevented by considerably higher forces of friction.

In accordance with a further feature of the invention, the frictionalforces are maximized by a suitable pairing of materials for the casingtube and the honeycomb bodies. Not only the surface properties of thematerials but above all their coefficients of thermal expansion play arole in that case. Although the casing tube expands very much more thanthe honeycomb bodies during a heat treatment or during the brazingprocess, the forces of friction abate in the process and undesiredshifting occurs.

In accordance with an added feature of the invention, althoughaustenitic special steels are very often used for the casing tube formetal honeycomb bodies, for the present invention it is advantageous touse ferritic corrosion-resistant special steel, especially steelidentified by material numbers 1.4509 (ASTM 441) or 1.4512 (AISI 409).

In accordance with an additional feature of the invention, even forcatalytic converters with a plurality of honeycomb bodies, the presentinvention makes it possible to use one-piece casing tubes, particularlywithout any round weld seam between the honeycomb bodies.

In accordance with yet another feature of the invention, in principle,all of the advantages of the invention can be utilized if the casingtube is graduated as well and if the individual honeycomb bodies havedifferent diameters.

In accordance with yet a further feature of the invention, the fixationand dimensionally accurate placement of the honeycomb bodies in a casingtube is reinforced by internal beads, which serve as a stop when thehoneycomb bodies are inserted and as a mount in the further productionsteps. It is entirely possible to compress a honeycomb body so far thatit can be thrust past an internal bead and then expand again somewhatbehind it. For many applications, however, it is possible to dispenseentirely with the additional provision of internal beads.

In accordance with yet an added feature of the invention, all of theprovisions described above may be employed unchanged if the honeycombbodies have different numbers of flow channels per unit ofcross-sectional area. They can also be employed for honeycomb bodieswith a different number of flow channels per unit of cross-sectionalarea distributed over the flow cross section.

In accordance with yet an additional feature of the invention, it isalso important that joint coating of a plurality of honeycomb bodies inone casing tube be possible. The lack of experience in this area isprobably one reason why thus-produced catalytic converters have thus farnot gained large-scale industrial use. It had initially been suspectedthat in coating with ceramic material, which is generally performed bysprinkling with a slurry of ceramic particles, difficulties could arisebecause relatively large droplets could form in the upper honeycomb bodyand would then plug the lower honeycomb body. In fact, however, and asdescribed in further detail below, it has been found that while meetingcertain peripheral conditions, uniform coating with ceramic andcatalytically active material even of a plurality of honeycomb bodies inone casing tube is possible in a single operation.

With the objects of the invention in view, there is also provided amethod for producing a catalytic converter, which comprises forming atleast two honeycomb bodies with a given theoretical strain-free diameterfrom structured metal layers defining channels through which a fluid canflow in a given direction; subsequently inserting the honeycomb bodiessuccessively or simultaneously with prestress from at least one sideinto a prefabricated casing tube having a given internal diameter;elastically compressing the honeycomb bodies by from 2 to 10% of thegiven theoretical strain-free diameter and/or subsequently plasticallycompressing at least partial regions of the casing tube by from 2 to 10%of the given internal diameter; and joining the layers to the casingtube.

In principle it makes no difference whether an elastically compressedhoneycomb body is introduced into an unchanged casing tube, or whetherthe initially slight elastic deformation of an honeycomb body isincreased after insertion into the casing tube by plastic deformation ofa casting tube.

Combinations of these two variants are also possible. In summary, onceagain, at least one prestress must be created that is equivalent to acompression by from 2 to 10% of the theoretical strain-free diameter ofthe honeycomb body.

In accordance with another mode of the invention, in order to ensurethat the honeycomb bodies can later be firmly brazed to the casing tube,it is advantageous to provide the casing tube with brazing powder on theinside, at least in some regions, before the honeycomb body is insertedor introduced, which is a provision that is known per se.

In accordance with a further mode of the invention, in a known manner,the outer end surfaces of the honeycomb body are acted upon by brazingpowder after they have been introduced or inserted into the casing tube.However, other methods for applying the brazing powder before or duringthe winding of the honeycomb bodies are also possible.

In accordance with an added mode of the invention, in order to increasethe mechanical strength of the entire configuration and to additionallyfix the honeycomb bodies, internal beads are pressed before or after thehoneycomb bodies have been introduced or inserted into the casing tube.These beads may serve as a stop and/or as a positionally accuratefastening for the honeycomb bodies.

In accordance with an additional mode of the invention, the formermethod steps assure that the prestressing of the honeycomb body is evenadequate for fixation during a high-temperature vacuum brazing process.The honeycomb bodies can therefore be brazed in an upright position in abrazing furnace, which in addition to saving space has other technicaladvantages. First, a more-uniform distribution of brazing material inthe circumferential direction is attained, because the brazing materialin the liquid state cannot converge on one side of the honeycomb body,and on the other deformation is avoided. The upright position can alsohave advantages for the sake of evacuating the furnace.

Even when coating is performed subsequently, the upright position isespecially advantageous, for similar reasons. In accordance with yetanother mode of the invention, in order to coat with a ceramiccomposition, the honeycomb bodies are sprinkled from above with a slurryof ceramic particles. As long as the slurried ceramic particles areconsiderably smaller in their particle size than the diameter of thechannels, coating of a plurality of honeycomb bodies located one belowthe other is possible without difficulty.

In accordance with yet a further mode of the invention, in honeycombbodies with a varying number of flow channels per unit ofcross-sectional area, the honeycomb body having the least number of flowchannels per unit of cross-sectional area is disposed at the top, sothat it is sprinkled by the slurry first. In that case, the distributionof the slurry is not substantially altered, nor do any relatively largedroplets form that could plug up channels located at a lower level in asecond honeycomb body.

In accordance with a concomitant mode of the invention, in the case ofgraduated casing tubes with honeycomb bodies of varying cross section,it is recommended that the honeycomb body having the largest crosssection be sprinkled from above, because this assures that the smallerhoneycomb body located at a lower level will also be completely coated,which could not be attained without difficulty if the configuration werereversed.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a catalytic converter with two or more honeycomb bodies in a casingtube and a method for its production, it is nevertheless not intended tobe limited to the details shown, since various modifications andstructural changes may be made therein without departing from the spiritof the invention and within the scope and range of equivalents of theclaims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly broken-away, diagrammatic, perspective view of acatalytic converter having two honeycomb bodies disposed in a casingtube;

FIG. 2 is a perspective view illustrating production of a honeycomb bodyhaving microstructures, extending transversely to a flow direction;

FIG. 3 is a cross-sectional view of a casing tube having three honeycombbodies with a varying number of cells per unit of cross-sectional area;

FIG. 4 is a fragmentary, cross-sectional view of a casing tube having aninternal bead for additional fixation of two honeycomb bodies;

FIGS. 5, 6 and 7 are fragmentary perspective and end-elevational viewsof a different configuration of a catalytic converter with two honeycombbodies; and

FIG. 8 is a cross-sectional view of a catalytic converter having twohoneycomb bodies of differing diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen a catalytic converterwhich includes a casing or jacket tube 1 with an internal diameter D,into which two honeycomb bodies 2, 3 are inserted. Both honeycomb bodiesinclude alternating layers of smooth metal sheets or layers 4 andcorrugated metal sheets or layers 5, which form many flow channels 6through which a fluid can flow in a flow direction S. An interstice orintermediate space 7 is free between the honeycomb bodies 2, 3. As issuggested by dashed lines for the honeycomb body 2, this body may bebrazed to the casing tube 1 in a specific circumferential zone 8. Thesame is naturally true for the honeycomb body 3. The honeycomb bodies 2,3, which are shown as being spirally wound for the sake of simplicity inFIG. 1, must be given a structure of their sheet-metal layers 4, 5, oran internal layout, in such a way that they are especially elastic. Theelasticity must be at least great enough to ensure that an initialstress-free or prestress-free wound body can be so severely elasticallydeformed that its diameter decreases by 2 to 10%. A diameter d of thehoneycomb bodies 2, 3 that would be established outside the casing tube1 in the strain-free state, as is shown in FIG. 2, would therefore becorrespondingly greater than the internal diameter D of the casing tube1.

FIG. 2 shows the basic layout of a spirally wound honeycomb body 3,which in the present case includes a primarily smooth sheet or layer 4and a corrugated sheet or layer 5. In the case of the present invention,the use of two markedly differently corrugated sheet-metal layers wouldbe even more advantageous, because it would produce more-elastic bodies.In order to additionally prevent slippage of the layers of the woundbody in the axial direction, microstructures 9 may be providedtransversely to the flow direction S in all of the sheet-metal layers.These microstructures catch in one another and as a result formform-locking connections, which increase the stability of the entirebody. Such microstructures 9 are also advantageous for influencing theflow in the channels 6 of the honeycomb body. As mentioned above, thediameter d of the honeycomb body 3 is suggested in FIG. 2. Themicrostructures have a height of up to 200 μm in the preferredembodiment.

The diagrammatic cross section of FIG. 3 shows a casing tube 11, inwhich three honeycomb bodies 12, 13, 14 having a differing number offlow channels per unit of cross-sectional area, are disposed.Interstices or intermediate spaces 17 remain free between the honeycombbodies 12, 13, 14. Such honeycomb bodies can be especially economicallymanufactured on a large scale in accordance with the present invention.

Specific requirements for volume and the number of flow channels perunit of cross-sectional area in the various honeycomb bodies 12, 13, 14can be flexibly adapted to given application conditions.

FIG. 4 shows a way in which an additional fixation for honeycomb bodies22, 23 can be created by means of internal beads 28 in a casing tube 21.An interstice or intermediate space 27 is free between the honeycombebodies 22, 23. In the example shown, the internal bead 28 serves as astop for the honeycomb bodies which are inserted from their endsurfaces. Particularly dimensionally accurate production is facilitatedthereby. However, identical internal beads on the end surfaces may alsoserve to fix the honeycomb bodies. Upon insertion, the honeycomb bodymust then be compressed so severely, that it fits past the internal beadand expands again somewhat after the internal bead. It is also possiblefor internal beads not to be made until after the insertion of thehoneycomb bodies 22, 23 into the casing tube 21.

FIGS. 5, 6 and 7 diagrammatically show a different possible layout for acatalytic converter including honeycomb bodies 52, 53 with sheet-metallayers 54, 55 intertwined in an S shape. Once again, these layers formmany flow channels 56. FIG. 6 shows the honeycomb body 52 with a lessernumber of flow channels 56 per unit of cross-sectional area. FIG. 7shows that the honeycomb body 53, which is merely shown in dashed linesin FIG. 5, has a greater number of flow channels per unit ofcross-sectional area. The theoretical strain-free diameter d of thehoneycomb body 53 is also suggested in FIG. 7. A coating 50 to beapplied to the individual sheet-metal layers 54, 55 is likewisediagrammatically suggested in FIG. 5.

FIG. 8 shows a further exemplary embodiment of a catalytic converter,which includes a first honeycomb body 83 that has a smaller diameterthan a second honeycomb body 82. Both honeycomb bodies 82, 83 areaccommodated in a casing tube, which includes one segment 81 of largediameter, one conical segment 84, and one segment 85 of smallerdiameter. The conical segment 84 has a free intermediate space 87.

Catalytic converters which are constructed and produced according to theinvention are especially suitable for economical large-scale massproduction, which is especially desirable for use in motor vehicles.Most known models, and many features known for improving such honeycombbodies, can be employed without difficulty in the catalytic convertersaccording to the invention as well.

We claim:
 1. A catalytic converter, comprising:a casing tube having a given internal diameter; at least two individual honeycomb bodies through which a fluid can flow in a given flow direction, said honeycomb bodies being disposed in said casing tube; said honeycomb bodies being formed of structured metal layers forming flow channels, said layers being joined to said casing tube by hard brazing; and said honeycomb bodies having a given theoretical strain-free diameter and having an elastic compression capacity of from 2 to 10% of said given theoretical strain-free diameter for prestress.
 2. The catalytic converter according to claim 1, wherein said honeycomb bodies have a structural form hindering axial shifting of said layers relative to one another.
 3. The catalytic converter according to claim 1, wherein said honeycomb bodies have internal form-locking connections hindering axial shifting of said layers relative to one another.
 4. The catalytic converter according to claim 1, wherein said casing tube is formed with one material and said layers are formed with another material for maximizing friction between said casing tube and said layers.
 5. The catalytic converter according to claim 4, wherein said casing tube includes ferritic corrosion-proof special steel.
 6. The catalytic converter according to claim 5, wherein said special steel is selected from steels having material numbers 1.4509 (ASTM 441) and 1.4512 (AISI 409).
 7. The catalytic converter according to claim 1, wherein said structured metal layers include many individual metal layers being intertwined with one another and each having ends being secured to said casing tube.
 8. The catalytic converter according to claim 1, wherein said honeycomb bodies include at least two types of differently corrugated layers.
 9. The catalytic converter according to claim 8, wherein said differently corrugated layers are wound spirally.
 10. The catalytic converter according to claim 1, wherein said layers have microstructures extending transversely or obliquely relative to said given flow direction with a height of up to 200 μm, for creating form-locking connections between said layers.
 11. The catalytic converter according to claim 1, wherein said casing tube is constructed in one piece without a round weld seam, between said honeycomb bodies.
 12. The catalytic converter according to claim 1, wherein said casing tube is graduated and has different diameters for said individual honeycomb bodies.
 13. The catalytic converter according to claim 1, wherein said casing tube has internal beads fixing positions of said honeycomb bodies.
 14. The catalytic converter according to claim 1, wherein said honeycomb bodies have different numbers of flow channels per unit of cross-sectional area.
 15. The catalytic converter according to claim 1, wherein said honeycomb bodies in said casing tube are coated with a ceramic composition and with catalytically active material in a single operation. 